The present invention relates to optical communications and more specifically to the restoring of optical pulses to remove distortion.
In optical communication networks, optical pulses carrying information bits undergo distortion for several reasons. First, dispersion in an optical fiber causes the pulse to spread temporally. Second, non-uniform amplification in optical amplifiers creates wings and humps in the pulse. Third, optical components in a network may have irregular transfer functions. Pulses may also be distorted in other ways. Distortion such as unwanted low intensity wings, which are added to high intensity pulse streams, degrade performance of an optical system and limit either data transmission bit-rate or network link length.
Such distortion is undesired as typical optical pulse widths or durations in optical communication networks are extremely short. For example, in optical carrier (OC)-48 systems transmitting data at a rate of 2.5 Gigabits per second (GB/s), the pulse width is about 400 picoseconds (ps). In OC-192 systems (10 GB/s), the pulse width is about 100 ps, and in OC-768 systems (40 GB/s), the pulse width is about 25 ps. Thus, higher data rates require shorter optical pulses, which suffer greater degradation in the time domain due to dispersion.
Presently, repeaters are provided along a network that acts as transceivers to convert optical pulses to electrical signals, restore the pulses by cleaning undesired artifacts and reshaping, amplify, and then retransmit them as optical pulses. In a metropolitan network such repeaters may be placed every few hundred meters to several kilometers (km) apart, whereas in a long-haul network, such repeaters may be placed every few kilometers to tens of kilometers. However, such repeaters raise network costs and complexity and do not fully remove distortion from the optical signals. In a wavelength division multiplexed (WDM) network system employing multiple wavelength channels, such pulse regeneration becomes very expensive since the individual channels must first be spatially separated using a demultiplexer, pulses restored, and channels recombined using a multiplexer.
A need thus exists to remove unwanted distortion from optical pulses and restore optical pulses by cleaning and shaping them to remove such distortion without the above drawbacks.